Apparatus for electrically eroding materials



March 12, 1957 E. M. WILLIAMS 2,785,279

APPARATUS FOR ELECTRICALLY ERODING MATERIALS Filed March 24. 1954 sShets-Sheet 1 {leer/900a 11 Z 3; i2 i 2 2:5 1+0 3 6 L- i6 1? 148 a y ifi3 g March 12, 1957 E. M. WILLIAMS 2,785,279

APPARATUS FOR ELECTRICALLY ERODING MATERIALS Filed March' 24, 1954 5Sheets-Sheet 3 United States Patent APPARATUS FOR ELECTRICALLY ERODINGMATERIALS Everard M. Williams, Pittsburgh, Pa., assignor, by mesneassignments, to Firth Steriing Inc., Pittsburgh, Pan, a corporation ofPennsylvania Application March 24, 1954, Serial No. 418,468

19 Claims. (Cl. 219--69) This invention relates to apparatus forelectrically dislodging particles from a conductive workpiece by aseries of short, time-spaced spark discharges, the art of so removingmaterials being sometimes termed as spark erosion or spark machining.

This application is a continuation in part of my previous and nowabandoned applications, Serial No. 201,- 657, filed December 19, 1950,and Serial No. 215,990, filed March 6, 1951.

Spark erosion or machining has found particular utility for machiningextremely hard materials such as tungsten and other carbides, hard steelalloys and the like. In boring operations, in tungsten carbideparticularly, this method has been proven many times faster than theconventional machining methods using diamonds or diamond dust and hasmade possible machining operations which could not be otherwiseperformed. Various other ad vantages are inherent in this type ofmachining, one being freedom from heat treatment problems as theworkpiece is not materially heated by the spark machining nor is thespark machining effectiveness afiected by the state of heat treatment ofthe workpiece. Another is the simplicity and inexpensive construction ofthe shaping tool since it serves merely as a non-contacting electrodewhile the sparks do the work of removing work- 1 piece particles orchips.

The inherent advantages of the spark machining process emphasize theproblem of improving spark machining apparatus for better realizingthose advantages. While higher current sparks at a greater repetitionrate are desired for faster cutting, it also is desired that the costand maintenance of the apparatus should not be unduly increased. Onedrawback in realizing this is the fact that the physical embodiments ofthe spark discharge circuits present such substantial reactance to therequired rapid current changes as to prevent effective utilization ofotherwise available spark machining energy.

It is therefore an obiect of my invention to provide an improved sparkmachining apparatus.

It is another object of my invention to provide a relatively simple andinexpensive spark machining apparatus capable of improved speed.

It is another object of my invention to provide a spark dischargecircuit for more effectively utilizing the electrical energy availablefrom the spark powering means.

It is a still further object of my invention to provide a physicalarrangement of the spark discharge circuit for improving its electricalcharacteristics.

The objects of the invention thus generally set forth, together withother objects and ancillary advantages are attained by the constructionand arrangement shown by way of illustration in the accompanyingdrawings, in which:

Fig. 1 is a circuit diagram of the charging and discharging circuit foruse in a typical apparatus embodying my invention.

Fig. 2 is a sectional view of the spark gap defined by 2,785,279Patented Mar. 12, 1957 the workpiece and electrode tool in a sparkmachining operation. I

Fig. 3 is a side elevation showing the mechanical arrangement ofapparatus suitably utilizing the discharge circuit of Fig. l andincluding the physical arrangement the work tank circuit.

Fig. 4 is a simplified representation of a modified work 'ank and worktank circuit.

Fig. 5 is a simplified representation of another modified work tank andwork tank circuit.

Fig. 6 is a representation of a discharge circuit and apparatus embodiedin my invention and particularly disclosing a coupled closed loop forminimizing inductance.

Fig. 7 is a schematic diagram showing a modification of the coupledclosed loop of Fig. 6.

Fig. 8 is a schematic diagram showing another modification of thecoupled closed loop of Fig. 6.

Fig. 9 is a view of a work tank together with both a coupled closed loopand a low inductance capacitor circuit.

Fig. 10 is a view of a work tank arrangement similar to that of Fig. 9but having a modified coupled closed loop structure.

While the invention is susceptible of various modifications andalternative constructions, there is shown in the drawings and willherein be described in detail certain preferred embodiments, but it isto be understood that it is not thereby intended to cover allmodifications, equivalents, and alternative constructions falling withinthe spirit and scope of the invention as expressed in the appendedclaims.

A better understanding of the operational requirements of sparkmachining apparatus generally and of the novel and distinctive manner inwhich those requirements are met by my invention as particularlydescribed hereinafter may be gained by first considering the generalarrangement of spark machining circuits together with the theory andoperation of spark machining.

Referring first to the general organization of spark machine apparatus,it may be seen from the circuit diagram of Fig. 1 that such an apparatusin simplified version comprises a direct current voltage supply 1 whichis connected through a charging network 2 to a storage circuit indicatedfor the purposes of general explanation as a capacitor 3. Theserepresent the basic components of the charging circuit. A dischargecircuit is provided by the cooperation of the capacitor 3 with a sparkgap 4 connected across the terminals of the storage capacitor 3, thespark gap being defined between a surface of a workpiece 5 and thefacing surface or surfaces of a spark machining electrode tool 6. Theelectrode is provided with controlled translational or rotational motionor both with respect to the workpiece by an electrode feed sys temrepresented schematically at 7. The spark gap is connected to thecapacitor 3 in such a way that the workpiece 5 is the anode or positiveelectrode and the tool is the cathode or negati e electrode of thedischarge circuit. This generalized circuit corresponds substantially tothe type disclosed in Patent 2,650,979 issued September 1, 1953, on theapplication of Edmund E. Teubner and assigned to the assignee of thepresent application.

The circuit is designed to repetitively store and discharge energy toproduce a series of short, time spaced spark discharges across the gap.Each discharge in the capacitative storage system occurs when it ischarged to that potential that produces disruptive breakdown of themedium between the spaced tool and workpiece. The ionization of thefluid medium in the spark gap be tween the electrode tool and theworkpiece is ideally tool, preferably made of brass, in a spark boringoperation is shown in Figure 2. It is important that the spark gap befilled with a dielectric liquid 8 such as kerosene, through which anionized current path is briefly maintained during disruptive breakdownupon sparkover. The voltage required to initiate the discharge increaseswith increase in the spark gap length, and with kerosene of a givenpurity or cleanliness the voltage drop required to maintain theionization during the spark discharge is in the vicinity of 30 volts.Practical spark gap lengths are small, being usually less than 'athousandth of an inch with brass cathode tools, and while the dischargepeak current may be in the order of thousands of amperes, the dischargeduration is usually at most a few hundred microseconds and preferablymuch less. The liquid dielectric helps provide a controlled sparkdischarge not only readily initiated at relatively ,low voltages, butalso readily deionized and thus terminated. It also servesnon-electrical functions such as conveying the entrained or suspendedremoved particles from the active spark gap region. As a matter ofdefinition of the kerosene or other fluid selected, it should beunderstood that the dielectric characteristics are such as to preventelectrolytic conduction of high currents of the order of magnituderequired for spark currents and that it has essentially liquid stateproperties as supplied to the gap The dislodging of particles from theconductive workpiece by the spark action, as presently understood by theapplicant, is best explained in terms of the electric field forceproduced by the spark current. Thus with the workpiece at a positivepotential with respect tothe electrode, the disruptive breakdown of thedielectric between them is the occasion for flow of electronic currentto the workpiece, Considering the spark terminus on the surface of theworkpiece as an approximate point source of current, the currentdensities at and just under the surface of the workpiece are very high.Due to the resistivity of the workpiece metal, a substantial electricfield gradient along the current path in the workpiece near the surfacereceiving the spark is produced. This electric field gradient results ina force on the positive ions in the material. These positive ions arethe atoms in the crystal lattice of the material less the associatedorbital electrons which are free to move and provide the ordinaryconduction properties of the material. The electrostatic forces on thatvolume of workpiece material thus positively charged tend to tear itaway from the main body of the workpiece. The rupturing forces mustovercome the tensile strength of the workpiece material to dislodge aparticle and thus erode or machine the workpiece.

Y Thesize of the fragment thus dislodged is limited by the decrease incurrent density with distance from the point current source on theworkpiece surface. Thus, neglecting skin effect, the current density Jrat any distance r from a point current cource on the workpiece surfaceis:

strength material, the radius of which this current density exists willtherefore be:

From this equation the radius, taken from the center of the ionizedpath, of the crater that occasioned the dislodgment of a workpiecefragment is proportional to the square root of the spark current.

The field force on the ions in the lattice of the electrode tool cathodeis directed into its surface rather than away from it so that the toolis not subjected to the rupturing forces on the anodic workpiece.

By the same theory the crater depth produced by a spark dependsprimarily upon the spark current. The time duration of the sparkdischarge, on the other hand, would seem most effective in increasingthe crater area, presumably since the spark terminus on the workpiecemay change somewhat due to the changing contour of the workpiece as thespark action proceeds.

Laboratory tests made with carefully controlled, very short dischargesof given voltage, yield the following empirical relation which is inaccord with the electric field force theory:

Where V is the volume of a particle crater, Kv is a constant includingthe relation of the tensile strength of the workpiece, T is the timeduration of the current, and I the current. Correspondingly for thedepth D of the crater:

where Kd is a constant corresponding to Kv.

Applying these relationships to spark cutting apparatus it is seen thatfor a given spark duration, the current amplitude determines the amountof material dislodged per spark, and that for a given amount of energy(prodnet of current and duration) effectiveness is increased byincreasing the current to time ratio.

The values involved in actual machining practice, as for tungstencarbide, for example, approach those predicted and confirm the analysis.Microscopic examination of the craters left upon particle dislodgementreveals evidence of fracture by mechanical forces. The crater left by asingle spark appears to be formed by the progressive removal of severalparticles or the progressive crumbling of the workpiece material. Inaccordance with the theory, the softness of a brass cathode electrodecompared to a workpiece anode such as tungsten carbide does not affectthe spark machining since it depends upon the electrical energy of thespark discharges and no physical contact whatsoever is made by theelectrode with the workpiece. The total material removal is, of course,the cumulative effect of a number of discharges and hence the repetitionrate is directly involved in actual practice.

Referring again to Fig. 1 a further explanation of the charging circuitis presented here for an appreciation-of the significance and functionof the physical embodiment of the discharge circuit.

As shown the direct current power supply 1 may suitably comprise astep-up transformer 9 having an adjustable tap primary winding connectedto any single-phase alternating supply voltage source It such as theusually conveniently available volt 60-cycle line. The ends of thesecondary winding are connected to the anodes of a pair of rectifyingdischarge devices 11 whose cathodes are connected in parallel to thepositive line 12. The center tap of the secondary winding is connectedto the grounded negative line 13. A filter network is preferablyprovided to reduce the voltage ripple of the rectified pulses andimprove the voltage regulation. Such a filter may suitably comprise twoinput chokes or inductive reactors 14 and 15 in series in the positiveline with by-pass capacitors 16 and 17 connected across the linesfollowing each choke. A conventional bleeder resistor 18 is connectedacross the second capacitor 17. For installations of a size involvinghigh average currents, other direct current power supplies than a singlephase fullwave rectifier may be advantageously substituted, and thefilter network may be modified or omitted as dictated by economicconsiderations.

The charging network 2 for the capacitor 3 is designed to permit rapidrecharging of the capacitor from the power supply 1 following initiationof the discharge of the capacitor stored energy in the spark gapdischarge circuit. To that end resistance and inductance are supplied tothe discharge circuit suitably in the form of a charging inductor 19 andcharging resistor 20 in the positive line. The time constant of theresistor-capacitor combination plays only a partial role in determiningthe charging time of the capacitor, in view of the inductance. Thus,during the spark discharge enough resistance is usually required tolimit the current flow directly from the power supply through the sparkgap, and allow an initiated spark discharge to deionize rather than todegenerate into a heating arc. At the same time the resistance increasesthe charging period and hence lowers the spark repetition rate. However,when the capacitor 3 discharges the instantaneous change of current flowthrough the inductor 19 is delayed because of its selfinductance. Thestored inductive energy is not released until after deionization of thespark discharge, thus permitting a significantly lower resistance valueof the resistor 29 to be employed than in the charging inductance wereabsent. The very delay in beginning recharging permits a much lower timeconstant and the charging rate is also subsequently increased by thereturn of the stored inductive energy to the charging circuit. Thischarging means is more fully described and claimed in the copendingapplication by Edmund E. Teubner, Serial No. 202,361 filed December 22,1950, and assigned to the assignee of the present invention.

The mechanical arrangement of the spark machining apparatus togetherwith a preferred physical arrangement of the discharge circuitcomponents in a work tank is indicated in Fig. 3.

The machine frame is suitably somewhat similar to a drill press, havinga pedestal base 21 and an upright support column 22 with a verticallypositionable radial support arm or platform 23 thereon on which ismounted the power transfer mechanism of the feed system 7. An electrode6 supported from the end of the radial arm 23 is raised or lowered withrespect to the workpiece 5 below for a spark machining operationanalogous to drilling. A vertical screw shaft 24 has an upper endportion threaded in a rotatable sleeve 25 journaled in the radial arm 23and a chuck 26 on the lower end of the shaft to hold the electrode tool.The screw sleeve 25 is rotated by a feed motor 27 to slowly lower theshaft and electrode tool and maintain the desired spark gap spacing asthe workpiece is eroded away. A manual adjustment control 28 geared tothe sleeve 25 is also provided. This feed control is only illustrative,and additional feed rate control means, including servomechanisrns, forexample, may be substituted, depending upon the requirements of theinstallation.

A work tank 29 mounted on the pedestal base 21 below the vertical shaft24 is employed to hold the workpiece in a bath of kerosene or otherliquid dielectric as is required for the spark machining operation.Since a major problem is removal of the particles or chips from thespark gap, the electrode tool 6 in this case is hollow, as is the screwshaft 24, in order that the kerosene and entrained particles may bepumped out of the gap and up through the tool itself. A pump 30, mountedon the arm 23 and having an intake line 31 coupled to the upper end ofthe shaft 24, suitably provides this function. The kerosene is returnedto the tank through a return line 32 which includes a filter 33 forcleaning the kerosene. Means for draining or storing the liquid may alsobe employed as desired to facilitate positioning of the workpiece in thetank. The tank or container size and shape may be varied to fit therequirements of the workpiece so long as the spark gap is inundated inthe dielectric liquid during sparking.

Considering now the discharge circuit, that is, the circui-t loop of thecapacitor 3 and spark gap 4, I have found that its inductance is alimiting factor in two respects. One is that the peak current of thespark is limited since inductive reactance is presented to the rapidcurrent change. The other is that oscillation of the current of thespark is caused. This is undesirable since undirectional electron flowacross the gap into the anodic workpiem is sought, and since oscillationdelays the deionizing of the gap and the recharging of the capacitor. Inaccordance with my invention I have found that elimination of lumpedinductance components is insufficient and that the inherent inductanceof the discharge circuit must be reduced as far as possible. Thus, byreducing the normal in ductance of the discharge conductors, thetransient or resonant response frequency is increased and the dampingincreased. While the values of inductance involved are very small, itsreduction is significant both in increasing the ratio of current to timein the first half cycle of spark current and the discharge repetitionrate.

In accordance with one aspect of my invention as shown in Figure 3, thework tank 29 serves as part of a low inductance discharge circuit forthe capacitor 3. Accordingly the work tank is preferably made ofconductive material, such as copper, or made of steel having an innercopper liner. In the example illustrated, the workpiece 5 is set up on abase plate 34 suitably made of copper and insulatingly supported fromthe floor of the tank by insulating support legs 35. The workpiece isalso insulatingly spaced from the tank walls, but the tank diameter issmall enough so that the walls are relatively close to the spark gapcircuit components. Dogs or clamps 36 of a suitable nature and size areprovided to fasten the workpiece 5 on top of the base plate 34 .and ingood conducting contact therewith. The capacitor 3 may suitably take theform of a high voltage oil filled capacitor having a metal casing whichserves as one of the electrodes and an insulating bushing on the upperend of the casing through which an upright terminal stud 37 is sealinglysupported. With this type of capacitor construction, the capacitor isplaced under the workpiece in conductive connection to the work tank bycontact of the under side of its casing with the inner bottom surface ofthe tank, but other capacitor types may be employed and connected inplace. The capacitor is preferably bolted in place to prevent thepossibility of an arcing contact by bolts through bottom capacitorcasing lugs or ears 38. A flexible conducting strap 39 suitably made ofcopper cable is connected between the upright terminal 37 and the lowersurface of the workpiece base 34 so as to effectively place thatcapacitor terminal in conductive relation with the workpiece 5 itself.

While capacitor units of different values may be employed or substitutedas desired, as is convenient, a number of capacitor units may bepositioned in the work tank and connected in series or parallel betweenthe tank and workpiece to provide the desired capacitance or voltagerating.

The connection of the other terminal of the capacitor 3 to the electrodetool 6 is provided by at least one flex ible conductor 40 between thetop of the work tank and the conductive electrode tool holder. In theconstruction shown, a plurality of such flexible conductive straps areconnected from a collar 41 conductively supported on the electrode toolholder to uniformly spaced regions on fastening flange 42 on the worktank rim. The flexible straps in eifect close the tank top to define avery low surge impedance, essentially coaxial transmission lineconnection between the capacitor, the workpiece and electrode arc erstool throughthe tankitself. By thus using the work tank itself as theouter conductor in the discharge circuit and eliminating all dischargeconductors outside'the work tank, the discharge 'circuitinductance isrcduced'to a fraction of what it would be if the receptacle 24 or itsequiva lent were omitted. The magnetic field is confined between theinner and outer conductors so defined and hence strong coupling isavoided. This shielding action also minimizes the opportunities forincrease in inductance due to magnetic-materials in proximity to thedischarge circuit. The tank, or at least its inner surface, isnon-magnetic, and magnetic coupling to the tank is also minimize Whilethework tank may take various configurations, the reduction ofinductance associated with coaxial lines is substantially retainedinsofar as the outer conductor or work tank carrying currentin onedirection surrounds the inner conductor or spark gap and associatedcircuit carrying current in the other direction. The form of capacitor,or indeed of any discharge current source, is not critical so long as itcan be coupled into-the enclosed circuit.

While the advantages of short lead length, and to that extent, loweredinductance, permitted by use of the work tank as a conductor are stillretained by using. for example, only one conductive strap 4 between theelectrode tool holder and the-tank top, the electromagnetic shieldingand that reduction of inductance due to the confining of the magneticfield in the work tank is forfeited. Accordingly, it is preferable thata plurality of conductors 40 be used so as to in efiect cover the tankso far as shielding is concerned. The shielding action is notsubstantially affected by the raising and lowering the electrode tool,there being enough slack provided in the flexible straps to accommodatethe degree of motion desired. It the electrode tool is also to be givena rotary motion as may be desired for such machining operations asthread cutting, the collar 41 to which the upper ends of the straps 4!)are connected is preferably made rotatable on the shaft 24, from whichthe electrode tool 6 is su ported. In order to afford good conductiveconnection between the sliding collar and the mach-inc shaft, the collarmay be shaped to provide-a hollow annular chamber surrounding the shaftand containing mercury. For a more complete escription of sucha mercurycontacting apparatus, reference is made to a copending patentapplication by Cecil P. Porterfield, Serial No. 402,675. filed January'7, 1954, and assigned to the assignee of the present inve tion, inwhich similar means are disclosed and claimed.

Charging circuit connections are suitably provided for the capacitor 3in the work tank 29. As shownthe negative or ground conductor may beconnected to a lug conductively secured as by bolting or welding to thework tank 29 and a positive conductor 44 is insulatingly sealed throughthe work tank wall at 45 to the stud 37 comprising the positivecapacitor electrode which is insulated from the capacitor casing andwork tank. These leads are, of course, subject to, or more likely tohave, a noticeable inductance according to their length and the mannerin which they are disposed A chassis or cabinet (not shown) containingthe elements of the charging circuit is preferably positioned on themachine column or placed near .to it. It is to be understood that sincethe inductance in the particularcharging circuit described is required,the value of the inductor 19 may be chosen with the value of inductanceprovided by the leads in mind so that the total inductance does notexceed the desired amount. As

good practice, however, it is desirable that the charging leads be shortto keep the inductance fairly low and thus minimize the significance ofcompensation or variations in compensation to be made for leadinductance.

lieferring now to Fig. 4, a modification of the low inductance work tankcircuit is shown therein, a capacitor 3 being schematically illustratedfor the sake of simplicity.

' A closed conductive non-magnetic work tank enclosure 46 is provided.Within the tank .46 is an open tray or inner tank 47 also made of aconductive metal. The

(ill

container 47 is supported by insulating spacers 48 from the bottom ofthe enclosing {tank 46. The workpiece 5 is suitably clamped in placewithin the tray 47 in conductive relation to it and the capacitor 3 isconnected between the tray and the work tank, being shown near thecenter of the tank between its bottom surface and under the tray. Thetank 46 is closed at the 'top by a removable conductive cover 49 havinga center conductive contacting bushing 59 through which the electrodetool 6 is admitted. Connections are suitably made to the capacitor 3from a charging circuit, the outer tank as being the negative terminaland a positive lead extending through an aperture 51 in the outer tankwall. The spark gap is maintained by sliding the electrode 6 through thetop bushing, the working :end of the electrode and the facing worksurface to be machined being submerged by the kerosene Or otherdielectric in. the inner container or tray ii. A vent opening 52 in thecover is desirable if the tank 46 is otherwise sealed to permit escapeof gases from decomposition of the kerosene.

With the construction exemplified "by Fi 4, a more complete enclosure 05the discharge circuit is Obtained, since the outer tank 46 is closedentirely around the capacitor 3. By such means the peak spark current isincreased and oscillation further damped. The tank cover 49 in this casealso provides a mechanical means to shield the operator from spatteringof the dielectric fluid caused by its turbulence at high cutting rates.The use of the separate inner tray 41 to confine the dielectric mediummay be eliminated if it is desired to use the single enclosure forconfining both the liquid and the magnetic fields of the dischargecircuit. Placing of the capacitor in the liquid dielectric also coolsthe capacitor and makes it possible to use a lower rate or smallerphysical size capacitor for a given average current.

Another modified work tank discharge circuit construction is illustratedin Figure 5 where the size of the capacitor 3 is large relative to thework tank 53, or it is otherwise impractical for the work tank tophysically encompass the capacitcr or capacitors. that while the voltageto which the capacitor is charged may be only in the order of a hundredvolts and the capacitance itself only a few microfarads, the physicalsize of the capacitor may be relatively large because of the highcurrent rates of change required which correspond to those of sinusoidalfrequencies very many times higher than the spark repetition rate.Referring still to Figure 5 the work tank 53 may suitably have anon-magnetic inner liner 54- made of more highly conductive materialthan the tank itself. The workpiece 5 is conductively secured to a baseplate 55 which is iusulatingly supported from the work tank by meansincluding insulating spacer 56.

Referring still to Fig. 5, the capacitor 3 below the work tank isconnected to it by a coaxial transmission line section 57, the linesuitably having an inner conductor carrying insulating spacer beadssurrounded by a flexible outer conductor 5?. The inner conductor 58extends through an aperture in the work tank and is connected to thebase plate 55. The other end of the inner conductor 58 is connected totie positive t rminal of the capacitor 3. The outer-conductor 5? has oneend connected as by soldering or welding to the work tank around theaperture for the inner conductor and has its other end connected to thenegative terminal of the capacitor 3. The capacitor 3 is suitablycharged through a charging circuit such as previously described. Asealing washer '69 in the work tank aperture prevents escape or leakageof the dielectric liquid into the coaxial transmission line section. Thedischarge circuit from the work tank to the electrode tool 6 iscompleted by means such as those described in relation to either Figure3 or Figure '4, a pair of c nductive V straps 61 being indicated in thedrawing. inlet and outlet tubing sections '62 and 65 are also shown asillustrative It Should be noted i of a manner in which the dielectricliquid may be circulated.

The transmission line section 57 shields that part of the circuit fromthe effect of magnetic materials or external circuits which mightotherwise be coupled through the magnetic fields associated with thedischarge circuit to increase its inductance. In effect, the outerconductor 59 in the transmission line section is an extension of theconductive work tank surrounding the inner lead to the capacitor. Theadvantages of the work tank as a currentcarrying conductor surroundingthe effective inner conductor carrying discharge current in the oppositedirection are retained.

Another manner of arranging the discharge circuit conductors for minimuminductance when the capacitor 3 for various reasons is not to beincluded in the work tank is indicated in Fig. 6. In this case a closedconductive loop 65 is arranged to couple the magnetic field produced bythe loop of the discharge circuit itself. It is to be appreciated, ofcourse that while no lumped inductance is present, some inherent ordistributed inductance is necessarily involved by reason of the areaenclosed by the circuit through which current flows when the spark gapdischarge occurs. Thus with a conductor 66 between the positive terminalof the capacitor 3 and the workpiece 5 and a conductor 67 between thenegative terminal of the capacitor 3 and the electrode tool 6, the loopthus defined has inductance to the extent that spacing betweenconductors 66 and 67 is involved. Inasmuch as the spark gap andcapacitor components have a material physical size, the self-inductanceof the circuit is significant in spark machining.

As indicated further in Figure 6, the conductor forming the shortcircuited loop 65 is formed to follow the contour of most of all of thedischarge circuit conducting path and to be closely adjacent theconductors 66 and 67 so as to enclose substantially the same area. Theloop conductor is suitably insulated as by an insulating tape andsuitably bound to the discharge conductors by tape or string. Sincemost, if not nearly all, of the magnetic field passing through thedischarge loop which generates it will also pass through the closed loop65, the energy coupled by the loop 65 is dissipated as an ER heat losstherein to thus effectively counteract or cancel discharge loopinductance. A similar effect is that of a transformer whose primaryinput inductance is reduced by short circuiting or heavy loading of itssecondary winding.

While the coefficient of coupling K as defined in the usual manner mayrange from .1 under only partial coupling conditions to about .9 withfavorable circuit arrangements, the higher degree of coupling is, ofcourse, preferable. The conductor of the loop 65 will have someresistance, but would normally have less than .005 ohm resistance forthe length of the conductor. This value may be considered as equivalentto a short circuit so far as inductance reduction for effective dampingis concerned. I

While the closed or short circuited loop 65 is shown as insulated, it isstill efiectively isolated if it makes contact with the dischargecircuit so long as the contact does not provide a conductive path aroundthe capacitor 3 or across the spark gap 4. Figures 7 and 8 illustratethe requirements. In these schematic representations of the dischargecircuit and the closed or short circuiting loop the direction of thedischarge current of the spark in the discharge circuit is indicated bythe arrows ii. The reverse induced current in the loop 55 or effectivesecondary winding is indicated by the arrows i2. if the closed loop 65merges with or is in contact with the primary loop along its negativeline between the negative terminal of the capacitor and the spark gap,as indicated by Fig. 7, or along its positive line between the positiveterminal of the capacitor and the spark gap, as indicated in Figure 8,the inductance canceling action remains the 10 same as if a fullyinsulated closed loop were employed. The net effect in the part of thecircuit carrying both the discharge current and the reverse inducedcurrent is to simply decrease the net flow of current in that portiononly of the discharge circuit.

In Fig. 9 a combination of the closed loop and coaxial work tankdischarge circuit is indicated in semi-schematic form. Thus a conductivework tank 68 provided with a top cover 69 is shown as the outerconductor surrounding an inner conductor including the spark gap 4 andthe capacitor 3. The capacitor is schematically indicated between theworkpiece 5 and the bottom of the work tank, being suitably connected toa charging circuit in the manner indicated with respect to embodiment ofFig. 3. In addition a short circuited or closed loop 70 is providedwithin the work tank. This loop may suitably take the form of theinsulated conductor 65 shown in Fig. 6. The loop lies in a radial plane,extending vertically along the inside wall of the tank, along the topand bottom, and vertically near the center vertical axis of the tankalong the spark gap 4 and the capacitor 3. in view of the coaxial natureof the work tank discharge circuit substantially all of the magneticflux of the discharge circuit is coupled by the loop 76. This may beseen to follow since the flux induced magnetic field is directedcircularly around the effective inner conductor. In cases where the worktank is especially large, the inductancecancelling effect of the loop 70may prove especially useful in further reducing the total inductance ofthe discharge circuit.

Since currents may be induced along closed paths in a conducting sheetor block having a region normal to the direction of the induced field,the closed loop need not be restricted in form to a loop outline. Amodification of the closed loop 70, Figure 9, is accordingly shown inFigure 10 to demonstrate one embodiment. Here again the capacitor 3 ispositioned in a work tank 7i between its bottom end and the workpiece 5,the electrode tool 6 extending from the top of the work tank to definethe spark gap 4 in cooperation with the workpiece. In this instance, byway of example, the capacitor 3 is shown in a tubular form, aligned withthe vertical center axis of the tank. The lower end 72 of the uprighttube is the negative terminal of the capacitor and in conductiverelationship with the tank, and the upper end 73 of the tubularcapacitor is connected to the workpiece. The positive and negativeterminals of the charging circuit are connected to the capacitor in themanner previously described.

The closed or short circuited-loop, of Fig. 10, preferably takes theform of a radially aligned conductive fin 74, suitably made of coppersheet. This fin is positioned between the top and bottom of the worktank and suitably has its outermost edge in conductive relationship withthe side wall of the tank while the inner radial edge of the member 74is spaced from the capacitor and spark gap, it being essential that itdoes not short either of them so as to maintain its effective electricalisolation or insulation from the discharge circuit. Again, as in themodification described with respect to Figure 9, the circular directionof the magnetic flux within the work tank induces reverse currents inthe fin 74. In this case, despite the fact that the loop is effectivelyclosed in by reason oi its being a solid conductive sheet, amultiplicity of closed induced current paths are established. Theshorter apparent discharge current path along the inner edge of the fin74 does not cancel the effectiveness of the fin since the coaxialconductor arrangement of the discharge circuit essentially determinesthe discharge current distribution and the magnetic flux pattern.

Summarizing briefly the improved discharge circuit arrangements, it isapparent that the inherent or distributed inductance is reduced by aconductor arrangement which distributes the current from the energyimpulse source through the spark 'gap so that'the magnetic fieldsproduced by the current impulses have a minimized inductive effect. Thusby utilizing the Work tank as a hollow conductor carrying current in theopposite direction from the remainder of the circuit it surrounds, theself-inductance is reduced for a given conductor spacing. By employing aclosed circuit magnetically coupled to the discharge circuit, theself-inductance is also reduced. As a result, the peak current isincreased, and the duration of the initial current impulse reduced andthe oscillation of the spark current is also damped more rapidly topermit more rapid recharging and hence faster repetition rates.

I claim as my invention:

1. Apparatus for electrically eroding a workpiece, said apparatusincluding means for supporting said workpiece, an electrode, means forsupporting and controllably moving said electrode toward said workpieceto maintain a predetermined gap therebetween, a condenser, a chargingcircuit connecting said condenser to a source of current, a dischargecircuit connecting said condenser to said workpiece and said electrode,and a low-inductive conductor connected in series in said dischargecircuit and having a form adapting it, in the presence of current theinductance of the discharge circuit whereby the peak discharge currentis greatly increased and its duration is greatly reduced.

2. The structure recited in claim 1 and a further conductor magneticallycoupled with, but electrically insulated from said discharge circuit forneutralizing a portion of the magnetic field linking said dischargecircuit by the induced current in the added conductor, further to reducethe effective inductance in said discharge circuit.

3. The structure recited in claim 1 in which said electrical conductoris in the nature of a receptacle enclosing said discharge circuit.

4. The structure recited in claim 3 in which the wall of said receptacleis relatively close to the component parts of said discharge circuit.

5. The structure recited in claim 3 in which said receptacleelectrically connects said condenser to said electrode.

6. The structure recited in claim 3 and a short circuited loop formed ofan electrical conductor and disposed within said receptacle, said loop'being magnetically coupled with, but electrically insulated from saiddischarge circuit.

7. In an apparatus for electrically dislodging particles from aworkpiece by passing a succession of electric spark discharges across adielectric-filled spark gap between the workpiece and an electrode tool,the combination comprising a conductive work tank for holding adielectric liquid, means for insulatingly supporting the workpiece insaid tank, means for lowering the electrode tool relative to theworkpiece to' define the spark gap therebetween, means for connecting asource of electrical impulses between said. tank and a workpiece in saidtank, and low inductance means for completing the discharge circuitconductively connecting the electrode tool to the top portion of thetank. 7

8. In apparatus for electrically eroding a workpiece having a spark gapdefined between an electrode tool and the workpiece, a source ofelectrical impulses, conductive means connecting said spark gap and saidsource in a series circuit, said conductive means being arranged toinclude a hollow conductive portion of said circuit surrounding an innerconductive portion including the spark gap.

9. In apparatus for electrically eroding a workpiece having a spark gapdefined between an electrode tool and the workpiece, a source ofelectrical impulses, conductive means connecting said spark gap and saidsource in a closed circuit, said conductivemeans including mean forreducing the magnetic field induced by a current impulse from saidsource whereby said circuit inductance is reduced.

10. In apparatus for electrically eroding a workpiece by sparkdischarges, a low inductance discharge circuit comprising a spark gapstructure defined by an electrode tool closely spaced with respect tosaid workpiece, a localized impulse voltage source, a substantiallyclosed hollow conductive body surrounding said source and said gapstructure, and means connecting said source and said spark gap structurein series circuit between opposite sides of said body to complete thedischarge circuit therethrough.

11. Apparatus forelectrically eroding a workpiece, said apparatuscomprising an electrode, a charging circuit having a condenser connectedacross the terminals of a source of direct current, a discharge circuitin which the condenser is connected across the gap between the electrodeand workpiece with its positive terminal connected 'to the workpiece andits negative terminal connected to the electrode, means for decreasingthe gap between the electrode and the workpiece, and an electricallyconducting closed loop magnetically coupled with but electricallyinsulated from the discharge circuit.

12. Apparatus according to claim 11, in which said closed loop has acoefiicient of coupling between about 0.1 and about 0.9.

13. Apparatus for electrically eroding a workpiece according to claim12, in which said closed loop has a substantially negligible resistanceand a substantially negligible leakage conductance with the dischargecircuit.

14. An apparatus for electrically dislodging particles from a workpieceby a succession of electrical spark dis charges caused to pass betweenthe workpiece and an electrode spaced therefrom by a dielectric filledspark gap, said apparatus'comprising, in combination, means for holdingthe workpiece, an electrode spaced from the workpiece by a spark gap,energy storage means including a condenser, a charging circuitconnecting said condenser for repetitive charging from a source ofelectric power, discharging circuit conductors connecting said condenserto the workpiece and to said electrode for the application of a seriesof time-spaced spark discharges across the spark gap, means foradjusting the size of the spark gap, and a low resistance closed circuithaving the conductors thereof adjacent said discharging circuitconductors for minimizing inductance in said discharge circuit wherebyeach such spark discharge has a substantially higher peak current ofsubstantially shorter duration than would be obtained in the absence ofsaid last recited means.

15. In an apparatus for machining a workpiece by the electricaldislodgment of particles therefrom, the com bination of means forholding the workpiece, an electrode tool disposable in spaced relationwith the workpiece with a dielectric filled spark gap situatedtherebetween, means for applying a series of time-spaced electricalspark discharges across the spark gap between the tool and theworkpiece, and closed circuit means magnetically coupled with theinherent inductance of said discharge applying means for increasing thepeak current and shortening the duration of each such spark discharge.

16. A method of machining a workpiece by electrical dislodgment ofparticles therefrom, said method comprising the steps of holding theworkpiece, bringing an electrode tool into spaced relation with theworkpiece so as to define a spark gap therebetween, maintaining thespark gap filled with dielectric fluid, applying by means of an electriccircuit a series of time-spaced electrical spark discharges across thespark gap between the tool and the workpiece, and controlling the peakcurrent and duration of each such spark discharge by controlling theinherent inductance of the discharge applying circuit.

17. The method of removing material from a workpiece by electricaldislodgment of particles therefrom, said method comprising the steps ofholding the workpiece, bringing an electrode tool into spaced relationwith the workpiece so as to define a spark gap therebetween, maintainingthe spark gap filled with dielectric fluid, applying by means of anelectric circuit a series of time-spaced electrical spark dischargesacross the spark gap between said tool and the workpiece, the tool beingcathodic and the workpiece being anodic, and increasing the peak currentand shortening the duration of each such spark discharge by the use of ashort-circuited conductor of negligible resistance magnetically coupledto but at least partially electrically isolated from the dischargeapplying circuit.

18. In an apparatus for electrically dislodging particles from aworkpiece by means of a succession of electrical spark discharges passedbetween the workpiece and an electrode spaced therefrom by a spark gapfilled with a liquid dielectric, the combination comprising a groundedconductive receptacle for containing the dielectric liquid and aninsulating support means for the workpiece, an

electrode spaced from the workpiece to define the spark gap, means formoving the electrode relative to the workpiece to maintain apredetermined spark gap length as the dislodging of particles proceeds,an energy storage means in the receptacle connected between saidreceptacle and said workpiece, a charging circuit connecting saidstorage means for repetitive charging from a source of electric power,and means for connecting said receptacle to said relatively movableelectrode to complete a low inductance discharge circuit from saidstorage means through said spark gap.

19. In an apparatus for electrically dislodging particles from aworkpiece by passing a succession of electric spark discharges across adielectric-filled spark gap between the workpiece and an electrode tool,the combination comprising a conductive work tank for holding adielectric liquid, means for insulatingly supporting the work piece insaid tank, a tool holder for lowering the electrode tool relative to theworkpiece to define the spark gap therebetween, means for connecting asource of electrical impulses between said tank and a workpiece in saidtank, and low inductance means for completing the discharge circuitincluding a plurality of flexible conductors connected between theelectrode tool holder and spaced points around the top portion of thetank.

References Cited in the file of this patent UNITED STATES PATENTS409,015 Cofiin Aug. 13, 1889 2,526,423 Rudortf Oct. 17, 1950 FOREIGNPATENTS 637,793 Great Britain May 24, 1950

